Recorded seismic data has a vertical axis in two-way time. In exploration areas in which subsurface geology is highly structured and the propagation velocity of seismic energy in the rock layers is highly variable, the time seismic is migrated in depth domain generating depth seismic, which has a vertical axis in depth, using a velocity model that provides an estimate of seismic velocity in the various rock layers. The depth seismic gives a more accurate image of the subsurface for the purpose of interpreting geologic structures, finding prospective hydrocarbon traps and planning a drilling program.
The goal of seismic interpretation is to quickly, easily, and confidently correlate what is seen in the seismic depth images with meaningful geologic events represented by data from existing wells in the area. In complex geologic environments, this can be challenging because the velocity model used to depth-migrate the seismic data is only an estimate of real subsurface velocities. As a result, the depths of seismic events seen in the depth seismic do not match the true vertical depths of corresponding geologic events seen in the well data. A depth-depth curve or mapping must be used to calibrate seismic depths to true vertical depths, thereby making it possible to correlate events seen in the depth seismic image to variations in rock layers as seen in the well data. Finding the correct depth-depth mapping is an iterative process in which the interpreter attempts to match corresponding events between the depth seismic and a synthetic trace (i.e. a model of a seismic trace computed from well data), which causes the depth-depth mapping to be updated resulting in a shift or stretch/squeeze being applied to the vertical axis of either the displayed well data and synthetic trace or the displayed seismic data. The interpreter then rechecks the match of well data and seismic data at all events of interest and repeats the depth-depth editing process as necessary.
Calibrating seismic depths to true vertical depths in this manner in order to correctly correlate seismic events to geologic events is challenging because of the uncertainty in the estimate of seismic velocities used to create the depth seismic and the possibility of mismatching seismic events to events in the synthetic trace. Consequently, it is critical to validate the depth-depth mapping to make sure that it makes geologic sense. Current methods used to validate depth-depth curves may produce values such as a percentage stretch over the various depth intervals in a depth-depth curve, or simply the difference between true vertical depth and seismic depth at each pair of depth-depth values in the curve. Neither of these quantities relate to geologic rock properties, which makes it difficult to validate the geologic reasonableness of the depth-depth mapping contained in a depth-depth curve using such methods.